A steel strip is generally hot-dip galvanized by: introducing a steel strip into a hot-dip galvanizing bath contained in a hot-dip galvanizing tank; upwardly reversing the travelling direction of said steel strip by a sink roll provided in the hot-dip galvanizing bath to introduce said steel strip through a pair of pinch rolls to outside said hot-dip galvanizing bath; and adjusting, immediately above the surface of the hot-dip galvanizing bath, the thickness of a galvanized layer deposited on the surface of said steel strip by a pair of slit nozzles ejecting a gas or any other appropriate means.
In the above-mentioned hot-dip galvanizing treatment, steel (Fe) composing the steel strip, the sink roll, the pinch rolls and other devices immersed in the hot-dip galvanizing bath very actively reacts with molten zinc (Zn). Dissolution of Fe into the hot-dip galvanizing bath is therefore inevitable. More specifically, Fe is dissolved into the galvanizing bath until the hot-dip galvanizing bath is saturated with Fe, i.e., until the Fe concentration in the hot-dip galvanizing bath reaches about 0.03 wt.%, and subsequently, precipitated in the form of an intermetallic compound of Fe and Zn (FeZn.sub.7). Said intermetallic compound (FeZn.sub.7), having the specific gravity of 7.25 which is larger than the specific gravity of Zn of 7.14, settles and accumulates on the bottom of the hot-dip galvanizing tank. This is why said intermetallic compound is usually known as "bottom dross".
The above-mentioned bottom dross accumulated on the bottom of the hot-dip galvanizing tank curls up through the hot-dip galvanizing bath under the effect of stirring of the hot-dip galvanizing bath caused by the ingression of the steel strip into the hot-dip galvanizing bath, and adheres to the surface of the travelling steel strip in the hot-dip galvanizing bath. As a result, particle-shaped protruding defects are caused by the adherence of bottom dross on the surface of the manufactured hot-dip galvanized steel strip, and seriously impair the appearance of the product.
In order to solve the above-mentioned problem, it has conventionally been a usual practice to temporarily discontinue the hot-dip galvanizing operation when the amount of bottom dross accumulated on the bottom of the hot-dip galvanizing tank reaches a certain value, and to discharge the accumulated dross by bailing it out from the bottom of the galvanizing tank with a bucket. In this method, however, not only the efficiency of discharge is very low, but also decrease in the productivity is inevitable because of the necessity of temporarily discontinuing the hot-dip galvanizing operation. The bottom dross bailed out from the hot-dip galvanizing tank contains Zn in a large quantity, and the recovery of Zn from the bottom dross requires re-refining of bottom dross. A plant not provided with dross re-refining equipment is obliged to sell the bottom dross bailed out from the hot-dip galvanizing bath to refiners, and cannot recover Zn contained in the bottom dross within the plant for reuse, thus leading to an increased zinc consumption.
For discharging bottom dross from a hot-dip galvanizing tank without interrupting the hot-dip galvanizing operation, a method is known, comprising adding aluminum (Al) into the hot-dip galvanizing bath. Addition of Al into the hot-dip galvanizing bath has conventionally applied in general also in an attempt to improve formability of the manufactured hot-dip galvanized steel strip. More particularly, an Fe-Zn alloy layer is formed in the galvanized layer of a hot-dip galvanized steel strip manufactured by hot-dip galvanizing. This Fe-Zn alloy layer, which is hard and brittle, causes, when working the hot-dip galvanized steel strip, breakage of the galvanized layer which results in peeling-off of the galvanized layer. With a view to preventing the above-mentioned Fe-Zn alloy layer from growing too thick and for improving the formability of the hot-dip galvanized steel strip, it is usual, in the hot-dip galvanizing operation, to add from above 0.10 to about 0.40% Al into the hot-dip galvanizing bath.
When Al is added into the hot-dip galvanizing bath, the above-mentioned bottom dross (FeZn.sub.7) reacts with Al as follows: EQU 2FeZn.sub.7 +5Al.revreaction.Fe.sub.2 Al.sub.5 +14Zn
The reaction given above proceeds from the left side to the right side at a free Al concentration of over 0.12 wt.% in the hot-dip galvanizing bath, and the bottom dross (FeZn.sub.7) accumulated on the bottom of the hot-dip galvanizing tank is converted into Fe.sub.2 Al.sub.5. This Fe.sub.2 Al.sub.5, having the specific gravity of about 4.5, which is smaller than the specific gravity of Zn of 7.14, floats up onto the surface of the hot-dip galvanizing bath. This is why Fe.sub.2 Al.sub.5 is generally known as "surface dross". The surface dross can be easily removed from the hot-dip galvanizing tank by scraping out even during the hot-dip galvanizing operation. Therefore, by converting the bottom dross into the surface dross through addition of Al into the hot-dip galvanizing bath so as to always give an Al concentration of over 0.12 wt.%, it is possible to easily remove the bottom dross from the hot-dip galvanizing tank without interrupting hot-dip galvanizing operation.
In an actual operation, however, it is not always easy to remove the bottom dross from the hot-dip galvanizing tank by converting the bottom dross into the surface dross through addition of Al into the hot-dip galvanizing bath and to keep the hot-dip galvanizing bath always in a state with the minimum bottom dross, for the following reasons.
More specifically, in an actual operation, a steel strip to be hot-dip galvanized is continuously introduced into the hot-dip galvanizing bath. The amount of bottom dross accumulated on the bottom of the galvanizing tank therefore increase gradually. In order to keep the amount of bottom dross in the galvanizing tank always at the lowest level, therefore, it is necessary to increase the amount of Al to be added into the hot-dip galvanizing bath. However, when increasing the amount of Al added into the hot-dip galvanizing bath, a reaction between added Al and steel (Fe) composing the steel strip, the sink roll, the pinch rolls and other devices immersed in the hot-dip galvanizing bath, takes place more actively than a reaction between Zn and Fe. Since a considerable portion of added Al is thus consumed in the reaction with Fe, the effect of Al addition to convert the bottom dross into the surface dross is reduced.
In order to keep the galvanizing bath always in a state of the minimum bottom dross in an actual operation, therefore, it is necessary to increase the amount of Al to be added into the hot-dip galvanizing bath, taking into account the amount consumed for the above-mentioned reaction with Fe. Addition of Al into the hot-dip galvanizing bath is usually effected by using a zinc ingot containing Al. In order to effectively prevent the production of bottom dross in the hot-dip galvanizing bath, the Al content in the zinc ingot should be at least 0.45 wt.%, and the free Al concentration of the hot-dip galvanizing bath should be kept at a high level of at least 0.20 wt.%.
However, when carrying out the hot-dip galvanizing operation of a steel strip with the use of a hot-dip galvanizing bath containing Al at such a high concentration, the following problems are encountered:
(1) Steel (Fe) composing the sink roll and the pinch rolls immersed in the hot-dip galvanizing bath actively reacts with Al contained at a high concentration in the hot-dip galvanizing bath, and is nonuniformly corroded. This causes serious irregularities on the surfaces of the sink roll and the pinch rolls, which in turn cause flaws on the surface of the steel strip and/or the surface of the galvanized layer thereof, thus resulting in a seriously impaired product appearance, and even in the impossibility of continuing the operation. PA0 (2) A large quantity of surface dross (Fe.sub.2 Al.sub.5) is produced in the hot-dip galvanizing tank by the reaction of Fe and Al as mentioned in (1) above. It is possible, as mentioned above, to easily remove the surface dross from the galvanizing tank by scraping out. However, a plant not provided with a re-refining equipment of dross is obliged to sell the surface dross, as in the case of bottom dross, to re-refiners, and cannot recover Zn contained in the surface dross within the plant for reuse. PA0 (3) A large quantity of Fe.sub.2 Al.sub.5 is produced in the form of a layer in the galvanized layer of the hot-dip galvanized steel strip manufactured with the use of a hot-dip galvanizing bath containing Al at a high concentration. When a large quantity of Fe.sub.2 Al.sub.5 is present in the galvanized layer, application of a galvannealing treatment (a treatment for converting the entire galvanized layer into a Zn-Fe alloy layer) to the galvanized layer of a hot-dip galvanized steel strip is impaired, and a uniform Zn-Fe layer cannot be obtained. When applying the galvanizing treatment, therefore, it is necessary to reduce the Al concentration in the hot-dip galvanizing bath in advance. PA0 (4) A chemical film serving as the primer is hardly formed on the surface of a galvanized layer containing a large quantity of Al, and it is impossible to obtain satisfactory paint adhesion.
Thus, it is very difficult, in an actual operation, to conduct hot-dip galvanizing with the use of a hot-dip galvanizing bath containing Al at a high concentration. This method is not therefore appropriate as a means to prevent production of the bottom dross.